Handover is one of the important aspects of any mobile communication system, in which the system tries to assure service continuity for the User Equipment (UE) by transferring the connection of the UE from one cell to another referred to as a Handover (HO). HO decisions usually depend on several factors such as signal strength, load conditions, service requirements, etc. The provision of efficient and effective handovers (minimum number of unnecessary handovers, minimum number of handover failures, minimum handover delay, etc.), may affect not only the Quality of Service (QoS) of the end user but also the overall mobile network capacity and performance.
In Long Term Evolution (LTE), UE-assisted, network controlled handover is used (3GPP TS 36.300, Third Generation Partnership Project Technical Specification No. 36.300). The network configures the UE to send measurement reports and, based on these reports, the UE can be handed over, to the most appropriate cell. A UE measurement report configuration consists of the reporting criteria (whether it is periodic or event triggered) as well as the measurement information that the UE is required to report.
In Evolved Universal Terrestrial Radio Access Networks (E-UTRAN), the decision to handover from the current serving evolved Node B (eNB) to a target eNB is made within the serving eNB and is made on the basis of measurements on the downlink (DL). These measurements are performed by the UE that measures the DL signals it receives from the different eNBs that it can receive.
The following event-triggered criteria are specified for intra-RAT (Radio Access Technologies) measurement reporting in LTE (3GPP TS 36.331):
Event A1: Serving cell becomes better than absolute threshold;
Event A2: Serving cell becomes worse than absolute threshold;
Event A3: Neighbor cell becomes better than an offset relative to the serving cell;
Event A4: Neighbor cell becomes better than absolute threshold;
Event A5: Serving cell becomes worse than one absolute threshold and neighbor cell becomes better than another absolute threshold.
The most important measurement report triggering event related to handover is A3, and its usage is illustrated in FIG. 1. The triggering conditions for event A3 can be formulated as the Equation below:N>S+(HS+CIOS,N)  (Equation)where N and S are the signal strengths of the neighbor and serving cells, respectively, HS is the hysteresis parameter that the serving cell applies for event A3, and CIOS,N is the Cell Individual Offset (CIO) set by the serving cell for that specific neighbor cell. If this condition is satisfied and it remains valid for a certain duration known as Time To Trigger (TTT), then the UE sends a measurement report to the serving eNB (e-UTRAN, evolved UMTS Radio Access, Node B). FIG. 1 shows signal strength on the vertical axis and time on the horizontal axis. A first solid line curve, shown high at time 0 is the signal strength of the signal received from the serving cell S. This is shown as declining in strength over time as the UE moves away from S and toward the neighboring cell N. The signal strength at N is low at time 0 and increases as the UE moves toward N.
The value for S+(HS−CIOS,N) is indicated by the dotted line in FIG. 1. Event A3 is satisfied at point A and a measurement report is sent at point B in time. Point A and Point B are separated by the TTT. When it gets the measurement report, the serving eNB makes a decision whether to initiate a handover toward the neighbor.
Handover in LTE is performed via an X2 connection, whenever available, and if not, using S1 (i.e. involving the Core Network (CN)). The X2 Handover process is shown in FIG. 2. The handover procedure can be sub-divided into three stages: preparation (initiation); execution; and completion. During the preparation stage (steps 1 to 3 of FIG. 2), based on the measurement results (step 2), the source eNB is getting from the UE, the source eNB decides whether to handover the connection to another eNB or not (step 3). Then a handover execution stage (steps 4 to 7 of FIG. 2) is entered and the decision to handover is communicated to the target eNB (step 4), and if the target eNB is able to admit the UE (step 5), a message is sent to the UE (steps 6 and 7) to initiate the handover. DL (Downlink) data arriving at the source eNB for the UE are then forwarded to the new target eNB (step 8).
The handover completion stage is entered once the target eNB and the UE are synchronized (steps 9 and 10) and a handover confirm message (step 11 of FIG. 2) is received by the target eNB. After a proper setup of the connection with the target eNB is performed (steps 12 and 13) (which include the switching of the DL path in the serving gateway, step 14, 15, 16), the old connection is released (step 17) and any remaining data in the source eNB that is destined for the UE is forwarded to the target eNB. Then normal packet flow can ensue through the target eNB.
As explained above, handover in LTE is controlled via several parameters. Incorrect parameter settings can lead to several problems such as (see e.g. 3GPP TS 36.902, 3GPP TS 36.300) radio link failure, ping pong handover, handover failure, etc.
A Radio Link Failure (RLF), is a failure that occurs when a radio link connection is lost for a predetermined time duration. For RLF, if the parameters are set in such a way that the UE does not report handover measurements on time, the UE might lose the connection with the original cell before handover is initiated. This is one example, of what is known as Too Late HO in which the UE tries to re-establish the connection with another cell after the RLF detection timers have expired. On the other hand, if the parameters are set to trigger handover very early, RLF might occur shortly after handover in the target cell. This is known as Too Early HO in which the UE tries to re-establish the connection with the source cell after the RLF detection timers have expired. Even if the handover is triggered at the right time, incorrect settings of the CIO can make the UE handover to the wrong cell, which is followed by a RLF and a re-establishment request in a cell other than the target cell or the source cell. This is known as HO to a wrong cell
In a ping pong handover, improper handover parameter settings can make the UE handover back and forth between two neighboring cells. An example of this is a setting that makes the triggering conditions for the handover events (A3) valid between the source and neighbor cells at the same time.
When the UE receives a certain number of (N310) consecutive “out of sync” indications from the physical layer, it assumes a physical layer problem is ensuing, and a timer (T310) is started. If the UE does not receive a certain number of (N311) consecutive “in sync” indications from the lower layer before T310 expires, RLF is detected. RLF is also detected when a random access problem is indicated from the MAC (Media Access Control) layer or upon indication that the maximum number of RLC (Radio Link Control) layer retransmissions has been reached.
Another type of failure is a HO failure in which the radio link between the UE and network was functioning correctly, but handover signaling messages failed to be exchanged. This might be due to congestion or because a maximum number of RLC (Radio Link Control) retransmissions was reached. When the UE receives an HO command (i.e. RRC Connection Reconfiguration Request with mobility Control Info, as shown in FIG. 2), it starts a timer (T304), and if this timer expires before the HO is completed (i.e. RRC Connection Reconfiguration Complete message is sent by the UE), an HO failure is detected.
When a RLF is detected by the UE, the UE starts a timer (T311) and tries to re-establish the connection to the best available cell (e.g. the source cell, another cell belonging to the same source eNB or a neighbor cell belonging to another eNB). When sending the re-establishment request (RRC Connection Reestablishment Request), the UE includes the following information (3GPP TS 36.331):
Global Cell ID (GCID) of the last cell the UE was connected to before RLF;
UE Identity: the Cell Radio Network Temporary Identifier (C-RNTI) as well as MAC ID for context lookup;
Re-establishment cause: whether the request is due to handover failure, reconfiguration failure, or other causes.
If the UE context is found in the cell (if it is the source cell or if it was a cell prepared for handover, (i.e. handover was ongoing when the RLF happened and the cell where the UE re-appeared already has the UE context, which was communicated to it from the source cell during Handover Request message exchange), the connection is re-established. Otherwise (if UE context is not available, or re-establishment did not succeed before T311 expires), then the UE has to go to idle mode and has to tear down all the active bearers, if any, and might restart the bearer setups, if required.
The eNB to which the UE is reconnecting, either through a successful RRC Re-establishment or via RRC Connection Setup after idle mode, can ask for more detailed information about the failure after the connection is completed via the UE Information Request procedure, where the eNB can ask for the RLF report. The UE responds by sending a UE Information Response message with a detailed RLF report which can include information such as (3GPP TS 36.331):
Measurement result of the last served cell before RLF;
Measurement result of the neighbor cells performed before RLF;
Location info, which can include last co-ordinates as well as velocity of the UE when RLF was detected;
CGI (and if that is not available Physical Cell ID (PCI)) of the cell where RLF occurred;
and, if the RLF occurred after the reception of a HO command (i.e. RRC Connection Reconfiguration message including the mobility Control Info), then also;
The CGI where this message was received;
The elapsed time since the reception of this message; and
The RLF type: i.e. whether it is a normal radio link failure or a handover failure.
Configuring all the HO parameters manually is expensive and can be very challenging. As such, Mobility Robustness Optimization (MRO) has been introduced in 3GPP to automate the dynamic configuration of handover parameters. Mobility Robustness Optimization (MRO) tries to gather statistics on the occurrence of Too Late HOs, Too Early HOs and HO to the wrong cell, and these statistics are used to adjust the handover parameters such as Hysteresis, CIO and TTT.
For MRO, the different HO problems discussed above are communicated between neighboring cells in different ways (3GPP TS 36.300, 3GPP TS 36.423, and 3GPP TS 36.331).
For Too Late Handovers, an RLF INDICATION message is sent via X2 from the eNB to which the UE tries to re-establish a connection to the eNB where the RLF occurred. The RLF INDICATION message contains:
Physical Cell Identifier (PCI) of the cell in which the UE was connected prior to RLF (known as failure cell);
E-UTRAN Cell Global Identifier (ECGI) of the cell where RRC re-establishment attempt was made;
UE Identity: C-RNTI and MAC ID of the UE in the failure cell; and RLF report (in a UE RLF Report Container Information Element (IE)).
If an eNB receives an RLF INDICATION message from a neighbor eNB, and if it finds out that it has sent a UE CONTEXT RELEASE message towards that neighbor eNB within the last Tstore_UE_cntxt seconds (i.e. it means that very recently the concerned UE was handed over properly to it from the same eNB), the eNB responds by sending a HANDOVER REPORT message that indicates Too Early Handover.
If an eNB receives an RLF INDICATION message from a neighbor eNB, and if it finds out that it has sent a UE CONTEXT RELEASE message towards another neighbor eNB within the last Tstore_UE_cntxt seconds (i.e. it means that very recently the concerned UE was handed over properly to it from another eNB), the eNB responds by sending a HANDOVER REPORT message that indicates Handover to the Wrong Cell.
The HANDOVER REPORT message contains:
Type of detected handover problem (Too Early Handover, Handover to Wrong Cell);
ECGI of source and target cells in the handover;
ECGI of the re-establishment cell (in the case of Handover to Wrong Cell); and
Handover cause (signaled by the source during handover preparation).
Thus, by analyzing the received RLF INDICATION and HANDOVER REPORT messages within a certain duration, eNBs can configure the optimal HO parameters to be used with their neighbors.
As mentioned above, current mechanisms such as MRO try to optimize mobility by fine tuning mobility thresholds such as CIOs with the objective of preventing further failures to occur.
Failures can also occur when a UE is moving at a high speed. LTE provides for speed dependent scaling of measurement-related parameters. UE speed information may be used to adjust cell reselection (cell reselection thresholds) and handover parameters (TTT). The UE can currently estimate its speed (high, medium, normal, known as the mobility state of the UE) based on a Mobility State Parameters configuration received from the eNB. The number of handovers in a given time is used to determine the mobility state as high, if there are many handovers, medium, if there are a medium level of handovers, and low, if there are few handovers in the given time. Thus, in the case of handover, the UE calculates its mobility state and the TTT may be adjusted accordingly by multiplying the TTT with a scaling factor associated with each mobility state.